The Internet Protocol (IP) Multimedia Subsystem (IMS), or IP Multimedia Core Network Subsystem, provides a framework for delivering IP multimedia services to devices. The wireless standards body 3rd Generation Partnership Project (3GPP) initially designed IMS to expand mobile networks beyond Global System for Mobile Communications (GSM) so that mobile networks could provide Internet services. For instance, 4G telecommunications technologies such as Long Term Evolution (LTE) (e.g., LTE Advanced, Evolved High-Speed Packet Access (HSPA+), etc.) can use IMS to deliver Internet services. Since then, multiple versions of, and/or updates to, IMS have been implemented by various entities so that IMS can be supported by various access networks and different communication technologies.
IMS uses Internet Engineering Task Force (IETF) protocols such as Session Initiation Protocol (SIP). Once attached to an access network such as LTE, a mobile device can initiate an IMS session (e.g., a VoLTE call) by requesting SIP registration from the IMS. During SIP registration, IMS requires the mobile device to establish a security association between the mobile device and the IMS (e.g., a Proxy Call Session Control Function (P-CSCF) server of the IMS) and/or an end-point device (e.g., a receiver device of the VoLTE call). A security association comprises security information (e.g., shared security attributes or “secrets”) between two network entities to support secure communication. For instance, the security association can include attributes such as: a cryptographic algorithm and/or mode to be used, encryption keys, and/or and parameters for the data to be passed over the connection (e.g., IP addresses). Consequently, the security association defines security mechanisms that bind IP addresses used to establish an IMS session so that communications via the IMS session are secure.
A problem with various access networks and/or the “mobility” of mobile devices is that connectivity interruptions to IMS sessions can occur. That is, a mobile device can lose connectivity to an access network (e.g., lose a coverage signal based on a change in location). Moreover, a mobile device can switch from one access network (e.g., LTE) to another access network (e.g., Wi-Fi) due to a change in coverage and/or a change in location. When these connectivity interruptions occur, the previously established security association is lost and the IMS session is subsequently terminated, thereby causing an inconvenience to the user (e.g., the VoLTE call is dropped). Consequently, the user experience is frustrated because the user has to initiate another IMS session (e.g., make another VoLTE call to the same person/device).